A conventional transversal filter used as a waveform equalizer automatically controls tab coefficients of delay elements having an inter-tab (tap) delay identical to a signal period. The filter is basically stable in view of the fact that it is of a non-cyclic type.
In a magnetic recording and reproducing apparatus for recording and reproducing a digital information signal through the use of a partial response method, there can be also used a waveform equalizer comprised of the transversal filter. In such a filter, the reproduced digital information signal is adaptively adjusted according to the difference between an output of the filter and an estimated value thereof in order to suppress the inter-symbol interferences of the reproduced digital information signal, the estimated value being obtained by mapping the output to one of predetermined ternary values. The outputs of the filter are then subject to a symbol decoder, e.g., Viterbi decoder, and an error correction device, e.g., RS (Reed Solomon) decoder.
In a reproducing circuit of the aforementioned magnetic recording and reproducing apparatus, as shown in FIG. 7, a reproduced signal, obtained by scanning a tape-shaped recording medium T (to be referred to as a "tape" hereinafter) with an aid of a magnetic head H installed on a rotary drum (not shown), is amplified to a predetermined level by a pre-amplifier (PA) 1 and noise components thereof are removed by a filter 2. The filtered signal is then converted into a digital signal by an analog-to-digital (A/D) converter 3. At a DC controller 4, a direct current (DC) level is set for the digital signal and the level adjusted digital signal is supplied as a reproduced digital information signal to a waveform equalizer E.
The waveform equalizer E includes delay circuits 11-14 for sequentially delaying and outputting the reproduced digital information signal fed to the waveform equalizer E; multipliers 15-19 for multiplying the reproduced digital information signals and the outputs of the delay circuits 11-14 by the outputs of lowpass filters (LPF's) 36-40 to be described later; an adder 20 for adding the outputs of the multipliers together 15-19 and outputting the result as an equalized output signal; a determination circuit F for virtually determining a value of the digital information signal by comparing the output of the adder 20 with predetermined thresholds; an error calculation circuit G for outputting an amplitude error of the equalized output signal with respect to its expectation by calculating the difference between the output of the virtual determining circuit F and the equalized output signal of the adder 20; multipliers 31-35 for multiplying the amplitude error by the reproduced digital information signal and the outputs of the delay circuits 11-14; and the LPF's 36-40 for lowpass filtering the outputs of the multipliers 31-35.
In multiplying the reproduced digital information signal and the outputs of the delay circuits 11-14 by the amplitude error of the error calculation circuit G, there occurs a delay in the amplitude error since the amplitude error is generated by the operation of the virtual determination circuit F and the error calculation circuit G. Therefore, a delay (not shown) is employed on the signal paths from the DC controller 4 and the delay circuits 11-14 to the multipliers 31-35, thereby synchronizing the multiplication timing of the multipliers, i.e., the amplitude error, and the multiplicands, i.e., the outputs of the DC controller 4 and the delay circuits 11-14.
In this connection, delay elements may be installed for the respective outputs of the DC controller 4 and the delay circuits 11-14 or the output of the delay 14 may be sequentially delayed to be supplied to the multipliers 31-35.
The reproduced digital information signal supplied from the DC controller 4 is sequentially delayed by a predetermined amount by each of the delay circuits 11-14 and then the reproduced digital information signal and the outputs of delay circuits 11-14 are multiplied by the outputs of the LPF's 36-40 at the multipliers 15-19, thereby obtaining outputs from the multipliers 15-19 weighted by tab coefficients. The multiplication results from the multipliers 15-19 are then added together at the adder 20 to be shaped for easy discrimination among signal waveforms having various values in order to obtain binary digital information data therefor. In other words, in case an original digital information signal waveform are resuppressed by inter-symbol interferences, the inter-symbol interference components of the corrupted waveform are suppressed by the waveform equalization carried out by adding weighted multiplication results obtained based on the tab coefficients from the LPF's 36-40.
The output of the adder 20 is supplied to the virtual determination circuit F and the error calculation circuit G. The virtual determination circuit F determines an expectation value of the output from the adder 20, by comparing the output with predetermined signal levels, the expectation value being one of, e.g., ternary values of -1, 0, 1. The determination result is provided to the error calculation circuit G, which calculates the amplitude difference or error between the output of the adder 20 and the expectation value, and provides same to the multipliers 31-35.
The multipliers 31-35 multiply the original signal, i.e., the reproduced digital information signal, and the delayed signals thereof (the timing is adjusted by the delay elements, as described above.) by the amplitude error, respectively. By performing the multiplication of the equalization error, i.e., the amplitude error of the equalized output signal, by the non-equalized input signals at the multipliers 31-35, there are obtained the tab coefficients to be used in equalizing the reproduced digital information signal. The waveform equalization of the reproduced digital information signal is accomplished by suppressing the inter-symbol interferences therein by multiplying the reproduced digital information signal and the delayed signals thereof by the tab coefficients.
As waveform equalizers for use in, e.g., a magnetic reproducing apparatus for reproducing a video signal recorded on a magnetic recording medium by using the partial response method and converting a video signal into a digital signal, there have been proposed a magnetic reproducing apparatus (disclosed in Japanese Laid-Open Publication No. 5-102793) having a filter for performing a simplified equalization on a reproduced signal depending on the characteristics of a pre-equalizer, thereby not requiring initial values for the respective tab coefficients of a digital filter acting as an adaptive equalizer and being capable of obtaining quick operation stability; and an automatic equalizing circuit (disclosed in Japanese Laid-Open Publication No. 5-291879) for automatically adjusting tab coefficients of an equalizer used for demodulation in digital data communicating or recording apparatus according to the distortion of an input signal and capable of being implemented by an analog circuit.
In the waveform equalizer described above, however, the determined value of the reproduced signal may result in an error value and then eventually diverge if initial values of the tab coefficients of the filter are not appropriate or inter-symbol interference becomes severe due to a deteriorated frequency characteristic of the input signal.
For instance, when a digital information signal shown in FIG. 8A is inputted to the waveform equalizer, the digital information signal is equalized to the one shown in FIG. 8B.
In FIG. 8C, sampling values of the information signal before waveform equalization are indicated in a negative direction and those of the information signal after waveform equalization are indicated in a positive direction. The sampling values correspond to either +1 or -1. It can be seen that positions of the sampling values before equalization do not completely coincide with those after equalization. In other words, unmatched sampling values are those erroneously recognized as +1's and -1's due to the divergence of the determined values. As a consequence, the eye pattern of the information signal obtained as shown in FIG. 8C converges into an erroneous value, as shown in FIGS. 9A and 9B, wherein FIG. 9A shows an eye pattern of an unequalized reproduced digital information signal and FIG. 9B illustrates an eye pattern of an equalized information signal whose determined value converges to the error value.
Conventionally, most of the transversal filters are formed by analog circuits, each including therein a plurality of delay circuits typically having time delays different from each other. In such a case, it becomes necessary to adjust the response of the individual equalizer or in some cases, an additional adjustable equalizer needs to be installed in front of the filter, resulting in bulky circuits and complicated adjustment thereof.
In the case of equalizing a signal having a transmission rates of several tens of MHZ, since sampling gates in the unit of several nano seconds (n sec) are generated for sampling signal waveforms, it is necessary to precisely adjust the positions of the generated gates, which is a troublesome task.
In a transmission system adopting a Viterbi decoder for recording and reproducing a digital information signal by the partial response method, there has been proposed by the inventor of the instant invention a waveform equalizer employing the transversal filter for sampling and removing a waveform error generated due to a distortion in a transmission channel by a bit clock used in Viterbi decoding, which does not, however, disclose any method of the solving the aforementioned problems (see Japanese Laid-Open Publication No. 6-303099).
A better equalization result can be obtained by improving the performance of the equalizer itself. Besides that, it might also be contemplated for the better equalization result to carry out the determination of a reproduced signal according to the results determined by using a circuit dedicated for performing the Viterbi decoding; however, the Viterbi decoding of the unequalized input signal does not guarantee a correct result either. In other words, since delay control is performed by the determination result of the Viterbi decoding after waveform-equalizing the reproduced signal, some information may be erased. Thus, it cannot be assured that the determination of the reproduced signal before equalization has been effectively performed.
Also, in the Viterbi decoder, it is necessary to maintain the determination results until the most plausible value is determined by a peak detection and, therefore, a relatively large memory is required. Thus, the system becomes costly and the configuration and the control thereof become complicated.
To be more specific, in the transmission path using the 2-bit digital delay correlationship in the waveform equalization of a digital information signal of a partial-response class-4 type, two memories are respectively used to perform discrimination of information signal values by an 1-bit interval, which can reduce the cost in connection with memory access works. However, according to this method, it may be difficult to determine appropriate digital values for the signal before equalization.
Also, in a Viterbi decoding and determination circuit, a faint input signal may not be waveform-equalized since threshold levels for virtually determining the digital information signal are set to be identical to those for determining final digital values.
The magnetic reproducing system (Japanese Patent Laid-Open Publication No. 5-102793) or the automatic equalizer (Japanese Patent Laid-open Publication No. 5-291879) supra does not offer any solution for such problems described above.